Gas damped crash sensors have become widely adopted by many of the world's automotible manufacturers to sense that a crash is in progress and to initiate the inflation of an air bag or tensioning of seat belts. Sensors constructed from a ball and a tube are disclosed in U.S. Pat. Nos. 3,974,350, 4,198,864; 4,284,863; 4,329,549 and 4,573,706 to D. S. Breed. A sensor constructed in the form of a rod with an attached coaxial disk, both arranged to move within a cylinder, is disclosed in the U.S. Pat. No. 4,536,629 to R. W. Diller.
Recently, it has been found that although the sensors disclosed in the Breed patents generally perform well during high speed crashes, their performance deteriorates significantly in marginal crashes especially when strong cross axis accelerations are present. One automobile manufacturer requires that the air bag not be deployed on crashes into barriers at 9 mph and always be deployed for crashes into barriers at 12 mph or above. When crash sensors are designed to meet this criterion, they perform well on laboratory shock test equipment. However, when placed on a vehicle and crash tested into a barrier at 12 mph the sensor frequently either does not trigger at all or triggers late. In the first case the occupant does not receive the protection of the air bag or belt tensioning device, and in the second case he/she is at risk of being injured by the deployment of the air bag.
It has been hypothesized and shown theoretically that there are some conditions where the sensing ball does not merely roll down one side of the tube but in fact undergoes a rather complicated whirling or orbiting motion. When this happens, a significant amount of energy is dissipated through sliding friction between the ball and the tube. This phenomenon has the effect of substantially delaying the motion of the ball and, on a marginal crash it can lead to a no-trigger or a late trigger condition.
This type of motion is caused by accelerations which are perpendicular to the longitudinal axis of the sensor tube. In the typical mounting arrangement, the sensor tube axis points toward the front of the vehicle and it is the accelerations in the vertical and lateral directions that can cause the whirling motion described above.
Cross axis vibrations have other undesirable effects, particularly on the electrical contact design currently used in gas damped ball-in-tube sensors. In particular, since the standard contact is a cantilevered beam, vibrations of the sensor can cause the contacts to vibrate resulting in several intermittent "tic" closures before solid contact is achieved. Similarly, when the contacts are first impacted by the sensing mass, (i.e. the ball) they frequently bounce one or more times. In one particular crash at 14 mph in which significant cross axis accelerations were present, the ball momentarily bridged the contacts causing a "tic" closure of insufficient duration to reliably trigger the air bag. Although this closure was on time, the air bag did not deploy until much later when a more solid contact closure occurred.
The ball-in-tube sensor currently in widespread use has a magnetic bias. Both ceramic and Alnico magnets are used depending upon the amount of variation in bias force caused by temperature that can be tolerated. Sensors used in the crush zone of the vehicle, and safing or arming sensors used both in the crush zone and out of the crush zone, can have ceramic magnets since they can tolerate a wide variation in bias force. Alnico magnets are used for the higher biased non-crush zone discriminating sensors where little variation in the bias can be tolerated. If a spring bias is employed in place of the magnetic bias as shown in the U.S. Pat. No. 4,580,810 to T. Thuen, the variation of the bias force with temperature can be practically eliminated. The use of a spring bias can also have the effect of reducing contact bounce and minimizing the effect of cross axis vibration on the contacts.
In the conventional ball-in-tube sensor, two cantilevered contacts are bridged by a gold plated ball. The gold plating is required to minimize the contact resistance between the ball and the contacts which are also gold plated. Gold is soft and easily damaged and the precise plating thickness and uniformity is difficult to control with the result that the dimensional tolerances of the ball can vary. This, in turn, affects the overall accuracy of the sensor. If the gold is eliminated from the ball, the cost of the ball may be substantially reduced and the accuracy of the sensor may be improved.
The thickness of the gold plating on the sensing ball is important from a corrosion viewpoint. A very thin coating of gold is all that is required to reduce the contact resistance. A thin coating, however, is porous and since gold has a different electromotive potential than the stainless steel ball, galvanic corrosion can take place if moisture is present in the sensor. Thus, a thick plating is preferred but this further increases the cost and reduces the dimensional accuracy of the ball. If the sensing mass, instead of bridging the contacts, is arranged to push one contact into another, the gold on the ball can be eliminated.
The U.S. Pat. No. 4,536,629 to R. W. Diller discloses a rod-in-cylinder gas damped crash sensor in which a contact spring is employed to provide a spring bias to the sensing mass. The U.S. Pat. No. 4,116,132 to Bell also uses a spring for bias. These sensors are also susceptable to contact bounce during operation.
The U.S. Pat. No. 4,329,549 to D. S. Breed discloses a biasing force of two to three G's applied to the sensing mass in a gas damped crash sensor. It indicates further that a biasing force of five G's need not be exceeded for such a crash sensor. However, a thorough study of vehicle crash libraries, has revealed that, when a crash sensor of this type is placed in the crush zone of a vehicle and is not located in the crush zone in certain marginal crashes, it will trigger too late and thus cause injuries to out-of-position occupants. For these marginal crashes, an occupant without deployment of an air bag will probably not be injured as seriously as by a late deploying bag. It is therefore desirable to make a crush zone sensor which does not trigger at all in these cases.